The invention relates to a magnetic field sensor comprising a layered structure E/F.sub.o /S/F.sub.f, in which:
E is an exchange-biasing layer, comprising nickel oxide; PA1 F.sub.o is a ferromagnetic layer with a fixed magnetization, comprising cobalt; PA1 S is a spacer layer; PA1 F.sub.f is a ferromagnetic layer with a free magnetization. For the sake of clarity, a number of terms in this definition will here be further elucidated: PA1 1. The terms "fixed" and "free" in relation to the magnetizations of the layers F.sub.o and F.sub.f are intended to have a relative meaning, viz. the magnetization M.sub.f of the layer F.sub.f is "free" with respect to the magnetization M.sub.o of the layer F.sub.o if, under the influence of an applied external magnetic field H, M.sub.f can be more easily rotated from its equilibrium position than M.sub.o ; PA1 2. The magnetization M.sub.o is "fixed" by exchange-biasing the layer F.sub.o to the layer E; PA1 3. The term "nickel oxide" refers to any stoichiometric or non-stoichiometric compound of nickel and oxygen. Although the symbol "NiO" will frequently be used in this context, this symbol should be viewed as encompassing compounds of the form NiO.sub.1.+-..delta., in which .delta. is a relatively small fraction. PA1 as magnetic heads, which can be used to decrypt the magnetic flux emanating from a recording medium in the form of a magnetic tape, disc or card; PA1 in compasses, for detecting the terrestrial magnetic field, e.g. in automotive, aviation, Maritime or personal navigation systems; PA1 in apparatus for detecting position and/or angle, e.g. in automotive applications; PA1 as field sensors in medical scanners, and as replacements for Hall probes in various other applications; PA1 as memory cells in Magnetic Random-Access Memories (MRAMs); PA1 as current detectors, whereby the magnetic field produced by such a current is detected. PA1 x+y+z=100, PA1 64.ltoreq.x.ltoreq.74 , PA1 15.ltoreq.y.ltoreq.22. Preferential examples of suitable such alloys include Ni.sub.66 Fe.sub.16 Co.sub.18 and Ni.sub.72 Fe.sub.21 Co.sub.7. For example, the inventors have observed that a test multilayer with the structure: PA1 Si(100)/50 nm NiO/3 nm Co.sub.90 Fe.sub.10/Cu/F.sub.f /Ta demonstrates an MR-ratio of 12.7% in combination with a free-layer coercivity of just 0.3 kA/m when the layer F.sub.f is comprised of 5 nm Ni.sub.66 Fe.sub.16 Co.sub.18.
Magnetic field sensors of this type may be employed inter alia:
A layered structure as described in the opening paragraph is known, for example, from an article by T. C. Anthony et al. in IEEE Trans. Magn. 30 (1994) pp. 3819-3821, in which the system NiO/Co/Cu/Co is studied. The authors report advantageously large room-temperature magneto-resistance (MR) ratios of up to 17% in this system (as compared to typical values of about 4-6% in conventional multilayers, i.e. multilayers using an FeMn exchange-biasing layer). However, the coercivity of the free Co layer is disappointingly high, with values of the order of 3 kA/m being observed in inner loop measurements (as compared to values of the order of 0.2-0.4 kA/m in conventional multilayers). Such high coercivity values greatly hinder potential use of the NiO/Co/Cu/Co system in practical applications.